Dietary protein is an essential nutrient for human health and growth. The World Health Organization recommends that dietary protein should contribute approximately 10 to 15% of energy intake when in energy balance and weight stable. Average daily protein intakes in various countries indicate that these recommendations are consistent with the amount of protein being consumed worldwide. Meals with an average of 20 to 30% of energy from protein are representative of high-protein diets when consumed in energy balance. The body cannot synthesize certain amino acids that are necessary for health and growth, and instead must obtain them from food. These amino acids, called “essential amino acids”, are Histidine (H), Isoleucine (I), Leucine (L), Lysine (K), Methionine (M), Phenylalanine (F), Threonine (T), Tryptophan (W), and Valine (V). Dietary protein sources that provide all the essential amino acids are referred to as “high quality” proteins. Animal foods such as meat, fish, poultry, eggs, and dairy products are generally regarded as high quality protein sources that provide a good balance of essential amino acids. Casein (a protein commonly found in mammalian milk, making up 80% of the proteins in cow milk) and whey (the protein in the liquid that remains after milk has been curdled and strained) are major sources of high quality dietary protein. Foods that do not provide a good balance of essential amino acids are referred to as “low quality” protein sources. Most fruits and vegetables are poor sources of protein. Some plant foods including beans, peas, lentils, nuts and grains (such as wheat) are better sources of protein but may have allergenicity issues. Soy, a vegetable protein manufactured from soybeans, is considered by some to be a high quality protein. Studies of high protein diets for weight loss have shown that protein positively affects energy expenditure and lean body mass. Further studies have shown that overeating produces significantly less weight gain in diets containing at least 5% of energy from protein, and that a high-protein diet decreases energy intake. Proteins commonly found in foods do not necessarily provide an amino acid composition that meets the amino acid requirements of a mammal, such as a human, in an efficient manner. The result is that, in order to attain the minimal requirements of each essential amino acid, a larger amount of total protein must be consumed in the diet than would be required if the quality of the dietary protein were higher. By increasing the quality of the protein in the diet it is possible to reduce the total amount of protein that must be consumed compared to diets that include lower quality proteins. Traditionally, desirable mixtures of amino acids, such as mixtures comprising essential amino acids, have been provided by hydrolyzing a protein with relatively high levels of essential amino acids, such as whey protein, and/or by combining free amino acids in a mixture that optionally also includes a hydrolyzed protein such as whey. Mixtures of this type may have a bitter taste, undesirable mouthfeel and are poorly soluble, and may be deemed unsuitable or undesirable for certain uses. As a result, such mixtures sometimes include flavoring agents to mask the taste of the free amino acids and/or hydrolyzed protein. In some cases compositions in which a proportion of the amino acid content is provided by polypeptides or proteins are found to have a better taste than compositions with a high proportion of total amino acids provided as free amino acids and/or certain hydrolyzed proteins. The availability of such compositions has been limited, however, because nutritional formulations have traditionally been made from protein isolated from natural food products, such as whey isolated from milk, or soy protein isolated from soy. The amino acid profiles of those proteins do not necessarily meet the amino acid requirements for a mammal. In addition, commodity proteins typically consist of mixtures of proteins and/or protein hydrolysates which can vary in their protein composition, thus leading to unpredictability regarding their nutritional value. Moreover, the limited number of sources of such high quality proteins has meant that only certain combinations of amino acids are available on a large scale for ingestion in protein form. The agricultural methods required for the supply of high quality animal protein sources such as casein and whey, eggs, and meat, as well as plant proteins such as soy, also require significant energy inputs and have potentially deleterious environmental impacts.
Accordingly, it would be useful in certain situations to have alternative sources and methods of supplying proteins for mammalian consumption. One feature that can enhance the utility of a nutritive protein is its solubility. Nutritive proteins with higher solubility can exhibit desirable characteristics such as increased stability, resistance to aggregation, and desirable taste profiles. For example, a nutritive protein that exhibits enhanced solubility can be formulated into a beverage or liquid formulation that includes a high concentration of nutritive protein in a relatively low volume of solution, thus delivering a large dose of protein nutrition per unit volume. A soluble nutritive protein can be useful in sports drinks or recovery drinks wherein a user (e.g., an athlete) wants to ingest nutritive protein before, during or after physical activity. A nutritive protein that exhibits enhanced solubility can also be particularly useful in a clinical setting wherein a subject (e.g., a patient or an elderly person) is in need of protein nutrition but is unable to consume solid foods or large volumes of liquids.